1. Technical Field
The present invention relates to a quarter wave plate used in optical devices such as an optical pickup device and a liquid crystal projector, or used in optical parts such as an optical lowpass filter. Especially, the present invention relates to a quarter wave plate made of an inorganic crystal material such as quartz crystal having birefringence and optical rotatory power. The present invention further relates to an optical pickup device and a reflective liquid crystal display device including the quarter wave plate.
2. Related Art
A phase plate having a quarter wavelength and converting a polarization state between linearly-polarized light and circularly-polarized light, namely, a quarter wave plate has been applied to various optical uses. As disclosed in JP-A-2005-208588, JP-A-2006-40343, JP-B-52-4948, and JP-B-3-58081 as first to fourth examples of related art, a quarter wave plate is commonly composed of a resin film which is made of an organic material such as polycarbonate and has obtained birefringence through a stretching treatment; a phase difference plate obtained by sandwiching a polymer liquid crystal layer by transparent substrates; or a crystal plate made of an inorganic crystal material such as quartz crystal having birefringence.
In recent years, in optical pickup devices used for recording and reproducing of an optical disk device, blue-violet laser having a very short wavelength and outputted in high-power is used so as to achieve high-density and large-amount recording. However, the resin film and the liquid crystal material described above have a property easily absorbing light in an ultraviolet region from blue. Therefore, the film and the material may emit heat by absorbing blue-violet laser light so as to deteriorate the material itself, disadvantageously deteriorating a function of the wave plate. In contrast, the inorganic crystal material such as quartz crystal has remarkably high light resistance, so that a quartz crystal wave plate is especially suitable for an optical system using blue-violet laser.
Quartz crystal wave plates having various structures have been developed. For example, a crystal plate of the third example is capable of highly-accurate conversion between linearly-polarized light and circularly-polarized light. The highly-accurate conversion is achieved by obtaining ellipticity k of main elliptically-polarized light and calculating thickness d of the crystal plate by the following formula when an optical axis of the plate is tilted from a normal line of an incident surface thereof.cos {(2π/λ)×Δn·d}=−{2k/(1−k2)}2 Here, λ denotes a wavelength of light incident on the crystal plate, and Δn denotes difference of a refractive index for the main elliptically-polarized light.
Commonly, in an optical pickup device, a laser beam from a semiconductor laser is converted from linearly-polarized light to circularly-polarized light by a quarter wave plate, and the light is reflected by a surface of an optical disk, then converted back to the linearly-polarized light by the quarter wave plate, and converted into an electrical signal at a light receiving device. However, the laser light is diverging light, so that a component, which is not converted into perfect circularly-polarized light but into elliptically-polarized light due to incident angle dependency thereof when the laser light passes through the quarter wave plate, is reflected by the surface of the optical disk to return to the semiconductor laser, disadvantageously causing unstable lasing. The fourth example discloses a quartz crystal plate which improves the problem of the incident angle dependency. The problem is improved in such a way that a refracting direction of extraordinary ray, which is determined by an incident angle of light, is set to agree with a crystal axis other than an optical axis and also set to be orthogonal to the optical axis so as to minimize an amount of change of phase difference between ordinary ray and extraordinary ray due to change of the incident angle. Accordingly, incident linearly-polarized light can be nearly-perfectly converted into circularly-polarized light constantly to be emitted.
JP-B-3-61921 as a fifth example discloses a phase plate as follows. The phase plate is formed by bonding two crystal plates to each other in a manner that optical axes of the plates are symmetrical to their bonding surface and are parallel to each other when viewed from a normal line direction of a plate surface, being able to cancel change of retardation caused by change of a beam incident angle. Further, JP-A-2006-40359 as a sixth example discloses a laminated quarter wave plate formed by bonding a first wave plate and a second wave plate. The laminated quarter wave plate exhibits a desired function as a quarter wave plate even when the first and second wave plates are arranged to be slightly tilted from an optical path because the plates are laminated in a manner to mismatch their optical axes for compensating disagreement of their optical axes caused by the tilted arrangement.
It is well-known that optical rotatory power of quartz crystal can influence capability of a quartz crystal wave plate. To solve such problem, JP-A-2005-158121 as a seventh example discloses a quarter wave plate which is less influenced by the optical rotatory power so as to improve properties thereof in a broad bandwidth. The quarter wave plate is formed by laminating two wave plates made of an optical material having optical rotatory power in a manner to make their optical axes orthogonal to each other and has such a structure that phase difference, an optical axis azimuth angle, optical rotatory power, and an angle, which is formed by a rotation axis and a neutral axis, of the wave plates that are obtained by an approximate formula using Poincare sphere satisfy a predetermined relational expression.
The quarter wave plate of the seventh example is described with reference to FIG. 26 showing a Poincare sphere same as that shown in FIG. 1 of JP-A-2005-158121. FIG. 26 illustrates an operation when light having a wavelength λ travels in quartz crystal. When a light incident direction is set to be a neutral axis S1 passing through two points Cf and Cs on the equator, phase difference Γ due to linear birefringence is given in a direction of the neutral axis S1, and phase difference 2ρ due to circular birefringence is given in a polar axis direction LR passing through the north pole and the south pole, a composite vector Γ′ is considered. When two points intersecting with the Poincare sphere on an extending direction of the composite vector Γ′ are set to be Pa and P, an angle β formed by a line PaP and a plane always including the neutral axis S1 and a neutral axis S2 which is orthogonal to the neutral axis S1 is expressed by the following formula.tan β=2ρ/Γ
Accordingly, the composite vector Γ′ is expressed as the following formula.Γ′=√{square root over (Γ2+(2ρ)2)}  Formula 3
Here, Γ and ρ satisfy the following relation when extraordinary ray refractive index is denoted as ne′, ordinary ray refractive index is denoted as no, right circularly-polarized light refractive index is denoted as nR, left circularly-polarized light refractive index is denoted as nL, and crystal thickness is denoted as d.
                              Γ          ⁢                      :                    ⁢                                          ⁢          phase          ⁢                                          ⁢          difference          ⁢                                          ⁢          due          ⁢                                          ⁢          to          ⁢                                          ⁢          linear          ⁢                                          ⁢          birefringence                ⁢                                  ⁢                  Γ          =                                                    2                ⁢                                                                  ⁢                π                            λ                        ⁢                          (                                                n                  e                  ′                                -                                  n                  o                                            )                        ⁢            d                          ⁢                                  ⁢                  2          ⁢                                          ⁢          ρ          ⁢                      :                    ⁢                                          ⁢          phase          ⁢                                          ⁢          difference          ⁢                                          ⁢          due          ⁢                                          ⁢          to          ⁢                                          ⁢          linear          ⁢                                          ⁢          birefringence                ⁢                                  ⁢                              2            ⁢                                                  ⁢            ρ                    =                                                    2                ⁢                                                                  ⁢                π                            λ                        ⁢                          (                                                n                  R                                -                                  n                  L                                            )                        ⁢            d                                              Formula        ⁢                                  ⁢        4            
Thus, the composite vector Γ′ is obtained by composing phase difference due to linear birefringence and phase difference due to circular birefringence, and can be handled as behavior rotating by a vector Γ′ on the Poincare sphere by using the line PaP as a rotating axis.
In the seventh example, in order to effectively simulate a phase difference characteristic in the quartz crystal wave plate having optical rotatory power, the quartz crystal wave plate is divided into n number of rotators Ti (i=1 or more) and n number of phase shifters Ri (i=1 or more) in a thickness direction. Thereby, a function W of the quartz crystal wave plate approximates as the following formula by using a determinant in which the azimuth rotators Ti and the phase shifters Ri function alternately.
                              W          =                                    T              n                        ⁢                          R              n                        ⁢                                                  ⁢            …            ⁢                                                  ⁢                          T              3                        ⁢                          R              3                        ⁢                          T              2                        ⁢                          R              2                        ⁢                          T              1                        ⁢                          R              1                                      ⁢                                  ⁢                  W          =                                    ∏                              k                =                1                            n                        ⁢                                                  ⁢                                          T                k                            ⁢                              R                k                                                                        Formula        ⁢                                  ⁢        5            
Especially, a quarter wave plate used in an optical pickup device of a high recording-density optical disk device is required to have high linearly-circularly polarized light conversion efficiency of ellipticity of 0.9 or more. However, the quartz crystal wave plates, described above, of the related art are designed without considering a direct influence on ellipticity and phase difference due to change of a polarization state of the wave plate caused by optical rotatory power of quartz crystal. Therefore, the influence of the optical rotatory power can not be totally excluded, so that it is difficult to make the ellipticity of the quarter wave plate have a high value of 0.9 or more or approximately 1.
The inventor inspected how the optical rotatory power influenced on the polarization state and how the change of the polarization state could be excluded in a quarter quartz crystal wave plate. First, ellipticity, phase difference, and thickness t of the wave plate in relation to a cutting angle of a quartz crystal plate were simulated under a wavelength λ of 405 nm, by using a quartz crystal wave plate which was designed by a common method to have an optical axis azimuth angle θ of 45° and design phase difference Γ of 90°. Here, the phase difference is actual phase difference generated between light incident on the wave plate and light emitted from the wave plate. The optical axis azimuth angle is formed by a polarization plane of linearly-polarized light incident on the wave plate and a crystal optical axis projected on an incident surface (or an emitting surface) of the wave plate. The cutting angle of the quartz crystal plate is formed by a normal line on the incident surface of the quartz crystal plate and Z-axis (the optical axis) of the quartz crystal. Here, right-handed quartz crystal was used in this simulation. The phase difference Γ is calculated by a well-known formula below.Γ=(360/λ)·(ne−no)t no: ordinary ray refractive indexne: extraordinary ray refractive index
FIGS. 27A to 27C are diagrams showing a quartz crystal quarter wave plate 121 and FIGS. 28A to 28C are diagrams showing a quartz crystal quarter wave plate 131. The quartz crystal quarter wave plates 121 and 131 are a single plate type which is well known. The wave plate 121 shown in FIGS. 27A and 27B has right-handed optical rotatory power for converting linearly-polarized light of incident light L into right-handed circularly-polarized light and an optical axis azimuth angle θ of 45°. The phase difference of 90° and rotation of a polarization plane due to the optical rotatory power act on the linearly-polarized light incident on the wave plate because of the birefringence of quartz crystal, so that the light is not emitted from an emitting surface as circularly-polarized light but emitted as right-handed elliptically polarized light as shown in FIG. 27C. The wave plate 131 shown in FIGS. 28A and 28B has left-handed optical rotatory power for converting linearly-polarized light of incident light L into left-handed circularly-polarized light and an optical axis azimuth angle θ of 135°. The phase difference of 90° and rotation of a polarization plane due to the optical rotatory power act on the linearly-polarized light incident on the wave plate because of the birefringence of quartz crystal likewise the above case, so that the light is not emitted from the emitting surface as circularly-polarized light but emitted as left-handed elliptically-polarized light as shown in FIG. 28C. The wave plate used in the present simulation had the structure shown in FIGS. 27A and 27B.
The results are shown in FIGS. 29A to 29C. Referring to FIGS. 29A and 29B, as the cutting angle φ of the quartz crystal plate is increased, the ellipticity approaches 1 and the phase difference is maintained at 90°. Thus an influence of the optical rotatory power is small. On the other hand, when the cutting angle φ is small such as in a range approximately from 5° to 20°, the ellipticity is less than 0.9, and the phase difference can not be maintained at 90°. As is apparent from FIG. 29C, when the cutting angle φ is in a range approximately from 30° to 90°, the thickness of the quartz crystal plate is thin such as approximately from 10 μm to 26 μm. Therefore, the strength of the quartz crystal plate is remarkably degraded, so that the quartz crystal plate is fragile and easily broken. Accordingly, it is difficult to handle the quartz crystal plate on manufacturing and on practical use.
In order to avoid the difficulty of the manufacturing of the crystal plate, the quartz crystal plate needs to have the thickness of at least approximately 80 μm. The cutting angle of the quartz crystal plate was set to be 10°, and wavelength dependency of phase difference and that of ellipticity were simulated. FIG. 30 is a graph showing a case where a quartz crystal plate is designed by a common designing method to have an optical axis azimuth angle θ of 45° and design phase difference Γo of 90°. The graph shows ellipticity by a solid line and phase difference by a dashed line. As shown in FIG. 30, the ellipticity was approximately 0.46 and the phase difference was 102.2° under the wavelength λ of 405 nm.
This is described with reference to a Poincare sphere of FIG. 31. In a case of FIG. 30, a reference point Po of incident light is set to as Po=(1, 0, 0), and a rotation axis Ro is set by being rotated by 2θ=90° from an S1 axis (positioned on an S2 axis) and then being tilted by an angle of 2ρ (ρ is an optical rotatory angle of the quartz crystal plate) with respect to an S1-S2 plane in a north pole (S3) direction. When the reference point Po is rotated clockwise by phase difference δ=90° around the rotation axis Ro, a point P1 on the sphere is an actual position of emitted light. Thus, elliptically-polarized light is emitted from a position largely apart from the north pole from which circularly-polarized light is emitted, thereby being unsuitable for an optical system such as an optical pickup device which demands high ellipticity.